Trying to associate commits with exercises.
Remove videos links from each lesson on home page.
Nicer-looking CSS for exercises in lesson plan.
Warn about odd numbers of indents.
Transform HANDOUT lines to look nicer.
Eliminate blank lines in lesson plans.
Add link from exercises in lesson plan to exercise page.
Make exercise formatting more readable.
Show links in monospace font (<code>).
Only show "relevant commits" header for exercises that have directories.